The present invention relates to a method and apparatus for the production of filter plugs which can be used in the manufacture of filter cigarettes, filter cigars and like smokers' products.
Filter plugs are normally obtained by subdividing a wrapped filter rod whose filler consists of a tow of filamentary filter material. The tow is treated with a plasticizer and is stretched to enhance the uniformity of resistance which the filter plugs offer to the passage of smoke.
U.S. Pat. No. 3,399,606 discloses an apparatus which measures the mass of filler material in the wrapped filter rod and the resistance to air flow across the tow upstream of the wrapping station. The stretching of the tow is regulated in dependency on the results of such measurements. A drawback of the patented apparatus is that the resistance which the tow offers to radial flow of air is not necessarily indicative of resistance which a filter plug offers to axial flow of the smoke. Another drawback of the patented apparatus is that the filaments of the tow are stretched below their elastic limit. As a rule, the degree of crimping of filaments varies unpredictably in longitudinal direction of the tow and a high degree of uniformity in the mass per unit length of the tow can be achieved only if the filaments are stretched until the crimp disappears. The operation of the patented apparatus is based on the premise that the stretching action results in partial elimination of crimp because, otherwise, the filaments would be extended beyond their elastic limit and the tow would break.